Electronic switch



Sept. 12, 1950 A. D. BROWN, JR

ELECTRONIC SWITCH 2 Sheets-Sheet 1 Filed May 8, 1946 m9 Im v am 4N maar? .vdo

INVENTORI D`N R MTU A A Y B sept. 12, 195o A. D. BROWN, JR

ELECTRONIC SWITCH Filed May 8, 1946 lags@ 2 Sheets-Sheet 2 -7 QGin control voltag e Patented Sept. 12, 1950 VUNITED STATES PATENT oFFIcE ELECTRONIC SWITCH Arling Dix Brown, Jr., Cambridge, Mass., assignor to the United States of America as represented by the Executive Secretary of the Oifce of vScientific Research and Development Application May 8, 1946, Serial No. 668,051)

3 Claims.

This invention relates to a method and circuit `arrangement for improved switching and more mechanical switch or a switch involving vacuum voltage for the two tubes. Although this method tubes, depending on the power and type of signal theoretically prevents any click, as any change involved. in current produced by a change in the bias volt- When switching an audio signal, not only age is balanced out, the diiiiculty of matching the should no transients be introduced by the switchtube properly and the lack of tubes suitable for ing but the actual change. in level should take a transformer coupling and bias gain controLmakes controllable length of time with no distortion of the circuit impractical for general use. the signal occurring, not only at the initial and An object of the invention therefore is to overnal states but durngthe change from one concome the difficulty referred to and to devise an dition to the other. The need for switching to improved method and circuit arrangement for be without transients `is obvious, in that any 2O audio switching. Other objects and novel featransients would produce audible clicks in the tures will appear from the following discussion. nal signal. The other requirement arises from In the accompanying drawings: the reaction of the ear which, although no actual Figure 1 is a schematic diagram of circuit electrical clicks or transients are present in the means for carrying out the method of the invensignal, will introduce an apparent click if the tion; signal is changed in level too rapidly. This can Figure 2 is a graph; and be explained by the fact that the sudden change Figures 3a, 3b, 3c and 3d are additional graphs in level spreads energy into other frequencies illustrating tube operation in accordance with than those existing in the signal; this added the invention. energy appears to the ear as aclick. The invention includes a vacuum tube called There are at least three possible methods of the signal tube, coupled as an amplifier, whose satisfying the basic requirements for audio signal gain can be controlled by a voltage which switching. The rst method is the manual or is referred to as the gain control voltage a second mechanical control of an attenuator. This is by vacuum tube called the balance control tube, far the simplest method but can be used only coupled to the gain control voltage so that an when a relatively long time interval for a change output voltage is obtained which is a function of in level can be tolerated, the gain control voltage; va third vacuum tube The second method will work only when sine known as the balance tube whose output is tiedl wave signals of a single frequency are present. to the output of the signal tube and coupled 'so Any electronic or mechanical method of rapid that the output current is a function of output switching is used and the controlled or output 40 Voltage of the balance control tube. signal is then fed through a narrow band pass The switching circuit is adjusted so that any filter centered on the same frequency as the sigchange in current in the output signal tube, nal. The output of this iilter, if the lter is caused by a change in the gain control voltage, is sufficiently sharp, will be free from both electrical balanced out by an equal and opposite current and apparent clicks. change in the balance tube. The result is an The third method is the use of vacuum tubes amplifier whose gain can be controlled by a to effect the switching. In general the signal is voltage change which cannot itself produce any fed into one grid of a tube and the output taken change in the output voltage. from the plate. The elective gain of the tube Considering the method of the invention in is then variedby controlling the voltage on the greater detail, it should be observed that it is same or another grid. ln switching other than similar to the rapid switching methods suggested audio circuits by this method, the switching is for other than audio signal circuits but uses more rapid and to avoid a transient as the tube goes elaborate circuits to insure a balance during the from cutoff to normal operating condition and entire switching period.

back, another tube is added whose plate is tied to the plate of the rst tube. The second tube is then controlled in the opposite fashion from the signal tube, i. e. when the one tube is conducting the other is cut off. Since the switching time is short, it is only necessary that the two tubes take the same current in their conducting condition to insure that no transient is produced.

In audio circuits it is customary to use a push pull circuit and again control the gain by a bias The first or signal tube is coupled as an ampliplate voltage of the signal tube.

er whose gain can be Varied by a control voltage. This control should preferably be a grid other than the signal grid and one which draws no current in order that the gain control voltage supply does not have to supply any power. Also distortion should be at a minimum at full gain, increasing as little as possible as the tube is turned from on to olf. The type 6L? tube satislies these requirements rather well with the No. 3 grid used for the gain control voltage and the No. 1 grid as a signal grid. The operation of the gain control voltage is shown in Fig. 3o. Changing the gain control voltage will cause a change in current in the tube independent of the eifects of the signal voltage. The additional two tubes are used to prevent this change of current in the signal tube from appearing as an output voltage.

The second (balance control) tube is also a 6L? whose No. 3 grid is coupled to the gain control voltage as seen in Fig. l.. Its plate is connected through a low resistance R3 to a regulated voltage supply of about the same voltage as the operating Since the @L7 acts as a pentode, i. e. the plate current is relatively independent of plate voltage, the changes in plate currents in the two tubes caused by changes in the gain control voltage, are about equal. The 1plate current characteristic is shown in Fig. 3b.

This change in plate current in the balance control tube causes a plate voltage change which is directly proportional to it, as given in Fig. 3c.

The balance tube, the third tube in the circuit, has its plate tied to the plate of the signal tube. It acts as a linear ampliiier, i. e. it produces a current change proportional to the voltage input. Since its plate resistance is a shunt on the 6L? load, it is desirable to use -a pentode, such as the GSJ?, which has a high plate resistance. The plate voltage changes in the balance control tube are applied through the voltage divider RiRz, which is used to balance out the steady direct current plate voltage of the balance control tube, to the grid of the balance tube. flihis provides a current change which is opposite and equal to the current change in the signal tube produced by the gain control voltage. The result, as shown in Fig. 3d, is that the steady current in the plate load remains constant regardless of the gain control voltage. Thus the gain of the signal tube can be varied from zero to maximum without the introduction of any transients in the output.

It is not necessary to use selected tubes in the circuit, because of the opportunity of using calibration potentiometers to compensate for slightly diiierent tube characteristics. Referring to Fig. l, there are four calibration potentiometers.

P1 for adjusting the bias on the No. l grid of the 6L? signal tube; P2 for adjusting the bias on the No. 1 grid of the 6L? balance control tube; P3 for adjusting' the screen voltage (grids No. 2 and No. 4) of the type 6L? balance control tube; and P4 for adjusting the bias on the type GSJ'T balance tube.

It is evident, referring to Fig. 2, that these adjustments effectively vary the cutoff voltage and the maximum plate current in the balance control tube, while maintaining the same characteristic shape of the curve for the tube when the plate current is plotted against the gain control voltage. Thus this tube is matched to the signal tube by the adjustment of these two voltages.

The calibration procedure is as follows:

The Ibias on the GSJ 7 balance tube is increased so that the tube is essentially cut oli. With the gain control voltage at O, the No. 1 grid bias on the 6L? signal tube is adjusted by means of Pi for 1.5 milliamperes plate current (of signal and balance tubes). With the gain control voltage at -7 or greater, i. e. signal and balance control tubes cut oif, the bias voltage on the 6SJ7 balance tube is adjusted by means of P4 to give 2.15 milliamperes plate current. As the gain control voltage is carried from -7 to 0, the No. 1 grid bias and screen voltage (grids No. 2 and No. 4) of the balance control tube are adjusted by means of P2 and P3 respectively to give minimum variation in the combined signal tube and balance control tube plate current from the 2.15 milliamperes value.

In addition to the switching circuit another electronic circuit is used in which a condenser C1 shunted by two diodes is used to give an approximately linear rate of opening and closing of the switch. The diode current characteristic olfsetting the non linearity of the charging and discharging of the condenser.

The duration of the opening and closing of the switch may be controlled by adjusting the values of C1 and R4 in Fig. l.

' It will beobserved therefore that I have provided a positive and eiiicient means oi clickless audio switching. While I have shown a preferred embodiment of the invention it is intended that various modications may be resorted to in keeping with the spirit of the invention as defined by the appended claims.

I claim:

l. Electronic switching` means for audio circuits comprising a rst vacuum tube adapted to function as a signal tube member and being coupled as an amplilier; Igain control voltage means for controlling the signal gain of the said signal tube; a second tube functioning as a, balance control tube, said second tube lbeing coupled to the gain control voltage to provide an output voltage which is a function of the gain control voltage; a third vacuum tube having its output tied to the output of the signal tube and coupled so that the output current is a function of the output voltage of the balance control tube; a variable resistor in series with the gain control voltage; a fourth tube functioning as a diode rectier; a variable condenser connected in shunt with said fourth tube; whereby said variable resistor, condenser, and said rectifier constitute a linearity control for said switching means.

2. Electronic switching means for audio circuits comprising a nrst vacuum tube adapted to function as a signal tube member and being coupled as an amplifier; gain control voltage means for controlling the signal gain of the said signal tube; a second tube `functioning as a bala-nce control tube, said second tube being coupled tothe gain control voltage to provide an output voltage which is a, function of the gain control voltage; a third vacuum tube having its output tied to the output of the signal tube and coupled so that the output current is a function of the output voltage of the balance control tube; a resistorin series with the gain control voltage; a fourth tube functioning as a diode rectier, a condenser connected in shunt with said fourth tube, whereby said fourth tube, said condenser and said resistor provide a linearity network for said switching means.

3. Electronic switching means for audio circuits comprising a vacuum tube adapted to function as a signal tube member and being coupled as an amplifier; gain control voltage meansfor controlling the signal gain of the said signal tube; denser and resistor comprise a linearity circuit a second tube functioning as a balance control for said switching means.

tube, said second tube being coupled to the gain ARLING DIX BROWN, JR. control voltage to provide an output voltage which is a function of the gain control voltage; a third REFERENCES CITED Vacuum tube haVng its Output tied t0 the out' The following references are of record in the put of the signal tube and coupled so that the me of this patent:

output current is a function of the output voltage of the balance control tube; a resistor con- UNITED STATES PATENTS nected in series with said gain control; a fourth 10 Number Name Date tube functioning as a rectifier having its anodes 2,092,496 Branson Sept. 7, 1937 coupled to the input circuits of the first and sec- 2,217,957 Lewis Oct. 15, 1940 ond tubes; a condenser connected in shunt with 2,360,466 Bedford et al Oct. 17, 1944 said rectifier, voltage, whereby said rectier, con- 

